(10) RFR (actually S) 8169881: Remove implicit Handle conversions oop->Handle (original) (raw)

Coleen Phillimore coleen.phillimore at oracle.com
Tue Feb 14 03:53:25 UTC 2017

On 2/13/17 8:44 PM, David Holmes wrote:

Hi Coleen,

"actually S"? :)

Well, maybe not but it shouldn't have a lot of merge conflicts for backports.

On 14/02/2017 6:10 AM, Coleen Phillimore wrote: Summary: Pass THREAD to Handle as argument instead of implicit Thread::current() call. Well there's a bit more to it than that as you also change parameter types so that we (sometimes) avoid: - oop -> Handle -> oop - Handle -> oop -> Handle

Yes, I put that in the bug but not in the RFR. The recommendation is to Handle the oop as far up as possible in the call stack and pass Handle around.

across method calls. This leads to some puzzling differences eg: src/share/vm/aot/aotCodeHeap.cpp src/share/vm/classfile/javaClasses.hpp The change in aotCodeHeap.cpp reverses the use of Handle and oop, but it is unclear why Throwable::print and Throwable::printstacktrace have different signatures in that regard. They used to both take a Handle but now one takes an oop instead. ??

Yes, you found the exceptions in Throwable. A lot of the java_lang_Throwable (and other javaClasses.hpp classes) pass oop instead of Handle and I changed print to oop to make it consistent, and because they it being called from other functions in Throwable with oop.
Throwable::print_stack_trace actually goes to a safepoint so has to be Handle.

More generally it is unclear why you made the changes you did in places - see below for some specifics. I'm left questioning how to know when to take an oop and when to take a Handle. Of course that question already exists, but these changes didn't make it any clearer for me.

Pass a Handle early and often is still the rule. For all but GC code.
There are still some functions that are very small and don't safepoint and take oop. Some of these have migrated to taking Handle, but some haven't. I didn't want to make this change larger for now.

I understand why the oop to Handle implicit conversion could be problematic due to the Thread::current() calls, but I don't understand why you couldn't keep (? add?) the Handle to oop implicit conversions ??

There weren't Handle -> oop implicit conversions. The conversion back to oop uses the operator() which remains.

The oop -> Handle conversion has cost, both at runtime and also to the GC because it increases the number of Handles on the Thread->_handle_area.

I did this same experiment years ago when Thread::current() was at the top of the profiling call stack but back then I needed to add too many explicit Thread::current() calls. There aren't many in this change, so it's worth doing. Also, metadata handles have the same problem but there are still too many of them to remove the implicit conversion.
And {instance}KlassHandle needs to be removed first. They do nothing now but hide the type.

--- src/share/vm/c1/c1Runtime1.cpp Aside: I'm a little surprised that there was not a Handle assignment operator or copy constructor such that given: 863 Handle appendix(THREAD, NULL); 970 appendix = cpce->appendixifresolved(pool); that it simply updated the NULL to the desired value, while keeping the THREAD intact.

Handle doesn't save the THREAD so a NULL handle is just oop* NULL. The RHS of the expression has to be converted to Handle and the default assignment operator copies it to appendix.

--- src/share/vm/ci/ciInstance.cpp Not at all obvious that replacing Handle with raw oop is correct. I'm not saying it isn't, just that it isn't obvious - and I'll wonder why the Handle was used in the first place.

It is safe because it's immediately unhandled in all the case statements, and I wanted to avoid a Thread::current call.

--- src/share/vm/classfile/javaClasses.cpp Why change javalangString::assymbolornull to take a Handle instead of an oop if you are simply going to unwrap the Handle to get the oop back. ??? Particular when a caller like src/share/vm/prims/methodHandles.cpp has to create the Handle from oop in the first place.

I changed it to be the same as java_lang_String::as_symbol(Handle java_string ...) which took a handle. java_lang_String::as_symbol() was the same - it immediately unhandles the argument to pass to the rest of the functions. I changed java_lang_String::as_symbol() to take an oop to match, and there are few scattered changes from that.

--- src/share/vm/jvmci/jvmciCompilerToVM.hpp It is not at all obvious that JavaArgumentUnboxers are thread-confined! I'm also concerned by API's (unfortunately a number pre-existing) that take a Thread/JavaThread argument but really require it to be the current thread. To that end I'd rather see thread as curthread and an assertion when it is set.

Since it's a ResourceObj and not StackObj (setting aside debate about that), I agree with you. I changed it to use Thread::current() where it handles the object, which it explicitly did before. In this case, it is a stack allocated thing but it's not guaranteed to be that.

--- src/share/vm/oops/cpCache.cpp Nit: objArrayHandle resolvedreferences (Thread::current(), ... Please remove space before (

fixed.

--- src/share/vm/prims/jvm.cpp Nit: need to fix indent on StackWalk::walk call second line.

fixed.

--- src/share/vm/prims/jvmtiGetLoadedClasses.cpp Ditto re thread -> curthread

This one is a StackObj (a Closure which should be used that way). So I changed _thread to _cur_thread and added an assert that it == Thread::current() in the constructor.

---

src/share/vm/prims/jvmtiImpl.cpp Nit: space before ( again (on handle constructor)

fixed.

It's a very small change to a number of files. Notably the new JVMCI files have a lot of implicit conversions from oop to handle. I want to get this change in early for jdk10 to stop this usage. I also added a few HandleMarks where I found that the HandlesArea was growing large. The addition and removal of the HandleMarks seems problematic because there are no obvious rules being applied. It is hard to know where and when a HandleMark is needed or not. Looking at src/share/vm/jvmci/jvmciCompilerToVM.cpp do we really want to create/destroy the HandleMark on each loop iteration? That would only seem necessary if we know the number of iterations is so high that we might consume all our handle space.

Nobody knows when and where to put HandleMarks. It's a dark art. I also was testing with a JVM that restored the assert if HandleArea got over 200 handles and the numbers got very high in these functions. The GC has to walk these HandleAreas and I am trying to reduce work for them.

I tried to write about this in the bug. I'd like to see if changing Handle to a proper constructor/destructor like methodHandles, so that only the Handles that are active need to be collected. Then we don't need HandleMark and their associated mystery. There are several HandleMarks in code that obviously doesn't allocate any Handles. I took out several but restored them so that this patch was smaller. To change Handle to a scoped object would require changing passing Handle as a const reference like I did with methodHandle which is too big of a change for early-jdk10.

The reason I worked on this RFE was because I was curious to see how many oops/Handles we're passing around and using from within the JVM these days. It's more than I thought.

Thank you for reviewing this. I'm going to rerun some tests with the changes above and send another webrev.

Coleen

Thanks, David ------

See bug for more on the motivation of this change.

open webrev at http://cr.openjdk.java.net/~coleenp/8169881.01/webrev bug link https://bugs.openjdk.java.net/browse/JDK-8169881 Tested by running all hotspot jtreg tests with -XX:+CheckUnhandledOops. There weren't any unhandled oops amazingly. Thanks, Coleen

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